Prior attempts that are presently known to have been disclosed in the field of this invention may be seen in U.S. Pat. No. 2,222,762. It was there intended that hollow bodies adapted for use as pressure vessels be produced from tubular metallic bodies by expanding the walls of the bodies to form a series of enlarged zones, such as spheroids, spaced by parts of the initial tube.
While this is a method of mass production, expensive control measures are required in order that the vessel walls have the appropriate thickness distribution. Otherwise considerably lower pressure resistivity than that available from optimal use of the material will result.
Other attempts at low cost production of cylindrical (metal) pressure vessels that have been noticed in the prior art include the U.S. Pat. No. 2,386,246. In this teaching it is desired to take two cylindrical shells each with one end closed and weld them together at their open ends. Thereafter heat is applied to the welded ends which are spun to a neck portion. This action thickens the closure and neck regions.
This patent teaches to apply a compressive axial force to thicken the neck. The purpose of the present invention is to cause a thinning as the radius is decreased.
In contrast to these known prior art attempts at obtaining metal containers this invention permits one to simplify the manufacture by taking a tubing of long length and doing identical, but optionally simultaneous, operations at a large number of stations. The stations, or areas, as general rule, extend for a length that is approximately equivalent to the diameter of the metal tube; and they are spaced apart a distance equivalent to several diameters. It is also intended to apply local tension to these areas. To avoid subsequent internal cleaning costs to remove oxides which form at elevated temperature if the metal is exposed to air, one may fill the tube with an inert gas such as argon or draw a vacuum to remove the air. The use of a long tube makes possible the forming of 2 necks at each station simultaneously.
This invention for the first time makes it possible to control the resulting thickness and associated radius distribution whereby a quality pressure vessel can be obtained. This control is based on the plasticity relations which apply for the given material as a function of temperature.
One may well attempt, upon an understanding of the present invention from the following description, to view this as an extension of glass forming techniques to the art of metal working. The differences are described below. The forming of glass is done when it is nearly molten, and it cannot support its own weight. A glass tube is sealed by relying on surface tension to draw the open end together. Glass vessels, made by somewhat similar means, do not and are not required to conform to the thickness requirements in a high quality pressure vessel. Finally, the use of constant strain-rate, stress-strain data is not taught in the fabrication of glass pressure vessels regardless of whether they are of appropriate high performance thickness distribution.
In conclusion then this invention has found two applications where it has filled a long standing need. One is in bringing to the art an economic manufacture of high performance pressure vessels; and the second is in bringing to the art the economic manufacture of low weight collapse resistant pressure vessels. Recently still a third possibility for this invention is in the improvement of the manufacture of a manifolded string of pressure vessels whereby one can have a far greater number of vessels with integral manifolds in a space than heretofore possible due to number of attachments, etc.